1. Field of the Invention
This invention relates to processes for shaping ceramic parts from powders, and molding compositions therefor. More particularly, the invention is directed to molding processes and molding compositions for forming high quality, complex parts which exhibit excellent green strength and which can be readily fired without experiencing the cracking, distortion and shrinkage problems commonly associated with prior art sintered products.
2. Description of the Prior Art
Several forming methods for ceramic bodies are commonly practiced. In one popular shape forming method, namely, slip casting, a liquid suspension of ceramic powder is "de-watered" in a porous mold, producing a powder cake in the shape dictated by the mold. Dry pressing involves compaction of a powder in a die. The powder usually contains a processing aid which serves as plasticizer and/or binder for the green compact.
One objective of any forming method is to produce green parts which can be sintered to a shape reproducible to close dimensional tolerances, free from defects During green-forming and sintering, cracks, distortions and other defects can arise due to the shrinkage associated with the particle consolidation processes. It is generally recognized that these defect-producing processes are mitigated by producing green bodies of high green density, which reduces the amount of shrinkage that the body must undergo during consolidation and sintering.
Another objective of shape-forming methods is to produce articles having net-shape, eliminating or minimizing the need for downstream operations, such as machining, to obtain final part dimensions. Dry pressing, in particular, frequently requires additional downstream processing in the form of machining and diamond grinding to attain intricate shapes, non-symmetrical geometrical formats and close tolerances.
Injection molding is recognized as a premier net-shape forming method for ceramic and metal powders. However, to realize the potential economic benefits offered by injection molding over other recognized less net-shape forming methods (e.g., dry pressing), it is necessary to minimize the number and complexity of the processing steps involved in the overall process. In addition, it is desirable that the molding compositions allow high solids loading to minimize shrinkage during binder burn-out and sintering to minimize defects as described above.
U.S. Pat. No.4,113,480 discloses the use of agaroid binders in forming ceramic or metal parts. The examples cited reflect relatively low solids concentrations, &lt;40 vol % of the formulations. Furthermore, the preferred embodiment of the process, as exemplified in the flow chart appearing in FIG. 2 of the drawings, teaches a sequence of steps including a step in which the batch formulation is concentrated in solids by evaporation of some of the water required in preceding processing operations. Evaporation of large amounts of water as a means of solids adjustment is a time and cost penalty, that in certain cases, renders the injection molding process uneconomical with respect to other shape forming processes.